The present invention relates to protection against hydrofluoric acid and it deals more particularly with various items of apparatus intended to contain hydrofluoric acid and comprising a metal wall coated with a material for protection against the chemical corrosion by hydrofluoric acid.
In the trade hydrofluoric acid is known in various forms which vary correspondingly in their corrositivies towards materials forming the walls of items of apparatus intended to contain it.
Anhydrous hydrofluoric acid is inert towards common steel, with the result that the problem of protecting the metal walls does not arise, provided that moisture, even in the form of traces, is prohibited from entering the apparatus.
Hydrofluoric acid called "purified 70%" contains sulfuric acid in order to inhibit steel corrosion. In fact, tests have shown that the addition of 3% of sulfuric acid allows hydrofluoric acid solutions to be stored and handled in contact with common steel, in a concentration of 70% or higher. The presence of the inhibitor (sulfuric acid) in hydrofluoric acid at this relatively high concentration is acceptable for some uses, for example for matting glass and for engraving porcelain and ceramics. On the other hand, in the case of economically very important uses such as the preparation of inorganic, especially uranium, fluorides, the separation of rare earths and the manufacture of electronics components, the presence of sulfuric acid to inhibit the corrosion is unacceptable.
Hydrofluoric acids known as "crystal 70%" and "crystal 50%" contain essentially ionized hydrofluoric acid in concentrations of 70% and 50% by weight respectively. These two forms are guaranteed to contain less than 0.03% of sulfuric acid. At this concentration there is no longer any inhibiting effect and the extreme corrosiveness of hydrofluoric acid towards metals is found, except for platinum, gold, silver and mercury, metals which cannot be chosen for forming a metal wall. Similarly, most so-called stainless alloys are rapidly attacked by these two solutions of hydrofluoric acid, so that the chemical industry is constrained to use only corrosion-resistant alloys which are very high in cost and tricky to fabricate, such as the alloys.
chlorinet 2, of composition having in percent by weight 63% Ni, 32% Mo, 3% Fe, 1% Si PA1 of composition having in percent by weight 60% Ni, 18% Mo, 18% Cr, 1% Si,
as well as certain Monel alloys based on nickel and copper, doped with other elements. It is found that even with these noble alloys, heavy-metal cations from the alloys transfer into solutions of hydrofluoric acid and subsequently act as contaminants in the processes where acids of "crystal" quality are employed.